Chapter 45. Percutaneous Nerve Localization

Conventional methods for peripheral nerve or plexus blockade have involved the identification of surface anatomic landmarks. Such landmarks serve as an approximate starting point for a search for the targeted nerve or nerves by needle exploration. The objective of needle exploration is to reach a finite endpoint that indicates the tip of the needle is sufficiently close to the targeted nerve or nerve plexuses. Two distinct types of endpoint exist.

1. 1. An anatomic endpoint based on encountering anatomic relations to the targeted nerve or nerves. Examples of blocks that make use of anatomic endpoints include field block, transarterial techniques, or ultrasonographic guidance.

2. 2. A functional endpoint based on a nerve response to mechanical or electrical stimulation. The main types of functional endpoints used clinically are either sensory response to mechanical stimulation of the nerve (mechanical paresthesia) or a motor response to electrical stimulation.

Designated anatomic landmarks are limited because they vary from patient to patient and do not always correlate with the location of the underlying nerve or plexus. In addition, traditional landmark measurements are sometimes complicated, requiring linear measurements with a ruler, bisecting lines, and they are not always normalized to patient size or body habitus. For many blocks, accepted descriptions of the technique include insertion of the block needle at a certain distance from a designated palpable landmark, without regard to patient size. Consequently, with many techniques, dexterity and delicate proprioception are often required for successful block performance.

Techniques such as ultrasonography or other imaging techniques or percutaneous localization utilizing transcutaneous electrical stimulation help to decrease needle exploration. Transcutaneous electrical stimulation, in contrast to an imaging technique such as ultrasonography, utilizes a functional neural response, either motor or sensory. Prelocalization of the nerve prior to needle insertion serves to decrease the amount of invasive search with the needle, increasing patient comfort while decreasing the potential for complications. The purpose of this chapter is to discuss how transcutaneous electrical stimulation helps to localize the underlying nerve or plexus through the skin, in a noninvasive manner before the needle is introduced transcutaneously. The chapter will necessarily discuss basic elements of nerve stimulation; however, for more in-depth coverage of principles of nerve stimulation and nerve stimulators, the reader is referred to Chapters 5 (Electrophysiology of Nerve Stimulation) and 17 (Equipment for Peripheral Nerve Blocks).

Elicitation of a paresthesia is an all-or-nothing phenomenon, ie, the needle either contacts the nerve or it does not. By contrast, use of electrical nerve stimulation yields graded information, which may be useful at a distance from the targeted nerve. Furthermore, visual cues of motor responses from untargeted nerves allow for redirection of the needle. This concept has been extended to the use of transcutaneous electrical stimulation to yield visual cues and motor responses, noninvasively, through the intact overlying skin.

Transcutaneous electrical stimulation to elicit a motor response has been used to assist in determining the optimal entry point for needle insertion, thereby narrowing the invasive search for the nerve with the needle. Ganta and colleagues1 reported on the use of a modified electrocardiographic electrode 0.5 cm in diameter with adherent coupling gel to assist in the performance of interscalene block. The electrode was coupled to a nerve stimulator and was “passed along the skin” to locate the optimal entry point for needle insertion. More recently, an exploring skin electrode was proposed to help find the interscalene groove in patients with difficult anatomy.2

Transcutaneous stimulation to elicit a sensory response (electrical paresthesia) to nerve stimulation of a purely sensory nerve (eg, lateral femoral cutaneous nerve) was described by Shannon and coworkers.3 These investigators used a small handheld electrical nerve stimulator equipped with 0.5-cm diameter metal electrodes to elicit sensory paresthesias of the lateral femoral cutaneous nerve. Following this, they made measurements to determine the approximate location of the nerve. Based on these measurements, injection of local anesthetic was made to block the nerve.

The objective of peripheral nerve location by electrical stimulation is to elicit a targeted motor response by a block needle coupled to a (square-wave) current generator (ie, nerve stimulator). The stimulator provides a stream of square-wave pulses, typically at a frequency (f) of 1 to 2 Hz. Ability to elicit designated motor responses below threshold current levels that have been empirically associated with high success rates indicates immediate proximity to the nerve.

Electrical Variables

The ability to electrically stimulate a peripheral nerve or neural plexus depends on:

• 1. Electric current amplitude (I), ie, the amperage applied to the stimulator electrode or needle
• 2. Pulse duration or width of the square wave of current generated by the pulse oximeter

And is inversely proportional to:

• 3. The distance between the stimulating electrode and the nerve
• 4. Tissue electrical impedance (mostly resistance) of the tissues that lie between and around the electrode and the targeted nerve or nerves

Current Amplitude (Amperage)

Use of higher amperage (eg, 2–5 mA) to stimulate peripheral nerves allows one to elicit a motor response at a greater distance from the nerve. As an electrode (the needle) approaches the nerve, motor response to electrical stimulation can be achieved at lower amperage (Figure 45–1). This is governed by Coulomb's law equation,

where E = required stimulating current, K = constant, Q = minimal required stimulation current, and r = distance between electrode and nerve.

Empirically, motor response to stimulation with current below 0.5 mA with pulse duration of 0.1 ms signifies that the needle's tip is sufficiently close to the nerve to translate to a high block success rate.

According to Coulomb's law, if a motor response can be elicited at very low amperage (<0.5 mp), then the stimulating electrode must be very close to the nerve. Stimulation at very low amperage maximizes specificity. In contrast, using a higher amperage (eg, 2–5 mA), maximizes sensitivity. This principle is used when monitoring the neuromuscular function by cutaneous electrodes and comparably very high (≈50 mA) currents during general anesthesia. Here, specificity of electrode location relative to the nerve is of less importance. For peripheral nerve location, 2- to 5-mA currents increase sensitivity, providing clues at a distance, but ultimate specificity is achieved by successful stimulation at very low current (<0.5 mA). However, it should be noted that using current of high intensity has practical disadvantages in that (1) it is associated with patient discomfort and (2) higher current intensity (eg, >1.0 mA) is sufficient to elicit direct muscle stimulation, which may produce confusing twitches of the local muscles. For these reasons, many nerve localization procedures are best initiated using relatively lower initial stimulating current (eg, <1.5 mA).

The surface area of the conductive electrode and its conductance are very important in nerve stimulation according to Ohm's law: I=V/R, where I = current flow (amperage), V = potential difference (voltage), and R = resistance (ohms).

The nerve stimulator varies current flow by altering the voltage. Most modern nerve stimulators are constant-current generators that automatically adjust the voltage output to maintain the set current flow despite changes in tissue resistance (within a certain range specified by the electrical design of the nerve stimulator).

Resistance, on the other hand, is related to the conductive area of the electrode by the equation R = ρ L/A, where R = electrical resistance, ρ = tissue resistivity, and A = conductive area. An example of the clinical importance of this principle is the use of defibrillating paddles. Defibrillation paddles are characterized by a large surface area to minimize impedance or resistance. By contrast, the microelectrode tip of a shielded block needle serves to maximize resistance outside of the microconductive area. This ensures that the electrode tip must be very close to the nerve if a motor response is to be elicited, thereby enhancing specificity.

If a motor response can be elicited at (1) very low amperage and, by Ohm's law, with a (2) microelectrode (small conductive area), then the stimulator electrode must be very close to the nerve. This phenomenon has led to the clinical use of needles with electrically insulated shafts to ensure specificity of location to the microelectrode (needle's tip) relative to the targeted nerve. Bashein and colleagues4 looked at the difference in the relative dispersion of current between electrically shielded and unshielded needles. Indeed, the ability to elicit a motor response to electrical stimulation following the initial elicitation of a mechanical paresthesia differs from 40%, using noninsulated needles, to 10% with insulated needles.5 The 30% increase in the ability to cause motor nerve stimulation with a noninsulated needle compared with an insulated needle can be explained by the difference in current dispersion between the two needles (see Chapter 5, Electrophysiology of Nerve Stimulation, for more information).

Electrical Pulse Duration

Electrical pulse duration refers to the duration of the periodic pulsed square wave generated by the nerve stimulator. For the purpose of nerve localization, short pulse duration ranging between 0.05 and 1 ms is typically used in clinical practice, with 0.1 ms being most common. Increasing electrical pulse duration increases the total flow of electrons (electrical energy) proportional to the calculated area under the curve (Figure 45–2). Increasing the duration of the electrical pulse therefore results in increased ability to stimulate the nerve without changing other variables. Similar to current flow (amperage), higher pulse durations of 0.3 to 1.0 ms also result in enhanced sensitivity for transcutaneous or initial invasive prelocation of the nerve. By contrast, by using a lower pulse duration, specificity is maximized. This principle has been demonstrated clinically when higher current amplitude (amperage) was needed to elicit a motor response at lower pulse duration (Figure 45–3).6

Tissue Electrical Impedance

Tissue impedance is a function of electrical resistance, capacitance, and inductance of the biologic tissue. In general, the higher the water–lipid ratio of the tissue, the lower the electrical impedance, or conversely, the higher the tissue conductance, R = ρ L/A, where ρ is the tissue resistivity.

Condensing the tissues by indenting the overlying skin, adipose, etc toward the nerve serves to decrease electrical impedance, increasing conductance.

Electrical Pulse Frequency

The frequency (f) of the square-wave electrical pulse generated by the nerve stimulator is typically set at 1 or 2 Hz. Increasing frequency to 2 Hz gives more rapid feedback with little added discomfort to the patient. Frequency must be sufficiently low so as to allow time for the relaxation phase period following depolarization. For example, frequency of 50 Hz causes sustained tetanus and is extremely painful and is therefore unacceptable for locating peripheral nerves for regional blockade. In addition, stimulation at frequencies greater than 3 Hz results in a loss of specificity; ie, motor response may be indistinguishable from a muscle fasciculations.

Percutaneous electrode guidance (PEG) of a block needle for peripheral nerve or plexus blockade is a recently proposed technique to assist with nerve location for regional anesthesia. PEG makes use of transcutaneous electrical stimulation to prelocate the nerve or plexus prior to needle insertion and exploration. The underlying motivation for the development of PEG was to devise a method to facilitate prelocation of the targeted nerve in order to minimize dependence on anatomic landmarks and measurements and to simplify the process of nerve location, particularly for the trainees.

Multiple Injection Peripheral Nerve/Plexus Blockade Via PEG

PEG allows for noninvasive, rapid identification of multiple superficial peripheral nerves. For example, all four nerves in the axillary brachial plexus (median, ulnar, radial, and musculocutaneous) can be located by stimulation and blocked (Figure 45–4).7 Multiple–injection techniques may result in a higher success rate with smaller volumes of local anesthetic. This is especially true in certain areas of a peripheral nerve plexus, eg, the axillary brachial plexus, where high compliance may lead to incomplete spread of anesthetic.8 For instance, Fanelli and coworkers studied multiple small-volume injection techniques for femoral–sciatic, axillary, or interscalene blocks in almost 4000 patients.9 Nerve stimulator–assisted separate identification and injection resulted in excellent (93–94%) success rates with low total local anesthetic volumes (22.6–28.1 mL). With axillary block, these investigators separately identified and injected the radial, median, ulnar, and musculocutaneous nerves. Interscalene block was performed following separate identification of supraclavicular, radial, and musculocutaneous nerves. Similarly, femoral nerve block followed separate elicited motor responses of the vastus medialis, intermedius, and lateralis components of the quadriceps muscles. Sciatic block was performed following separate motor responses of foot flexion, extension, and biceps femoris contractions. Percutaneous electrical guidance is a promising technology for noninvasively prelocating and aiding in these multiple injection techniques.

Initial Experience with PEG

In the initial report, a cylindrical electrically shielded cutaneous electrode with a 1-mm diameter metallic conductive tip was used.7 The probe was positioned over the skin overlying the nerve. Maximal motor response at minimal cutaneous probe amperage (2 Hz, 0.2 ms) was taken as evidence that the probe was directly above the nerve. A standard commercial nerve stimulator needle was then physically guided by the probe to the nerve. Block characteristics are shown in Table 45–1. Since nerves were prelocated by cutaneous probe, needle insertion in all but one case was made with a small current intensity of 0.5 mA. The milliamperage of a minimal transcutaneous stimulation current correlated directly with measured needle depth (beyond the probe tip) when minimal stimulating needle current occurred. Following the original publication of the PEG technique, a significant progress in improving the probe and simplifying the technique was made.10

Capdevila and colleagues11 published a larger study in which the needle tip itself was used to transcutaneously and noninvasively locate the nerves of the axillary brachial plexus. This was accomplished at nerve stimulator settings between 5 mA and 0.5 mA (0.2-ms pulse duration) prior to needle insertion. These investigators, however, found a linear relationship between the needle depth at final location and minimal transcutaneous stimulating current (in milliamperes) (Figure 45–5). Subsequently, a report of PEG to perform femoral nerve block in obese patients without palpable arterial pulse was published by Pham Dang and coworkers.12 These authors used a metallic probe to indent the skin and stimulate the nerve (which required up to 8 mA, 0.5-ms pulse duration). The skin was marked, and the standard block needle advanced using the skin markings as a guide. Although this method does not entirely represent PEG in that the needle was not physically guided by the transcutaneous stimulating electrode, it exemplifies the value of the concept. The technique used by Pham Dang and coworkers would be more correctly termed transcutaneous prelocation using a cutaneous electrode and is useful in determining the site of needle insertion.

In summary, conventional methods for peripheral nerve or plexus blocks have relied on anatomic landmarks and needle exploration. Percutaneous localization allows prelocation of nerves by transcutaneous stimulation before invasive needle exploration. Electrical variables that include current flow or amplitude, pulse duration, and tissue impedance can be manipulated to enhance sensitivity and specificity for optimal transcutaneous nerve location. Percutaneous electrode guidance of the block needle can be used to facilitate superficial nerve or plexus blockade, or when more traditional palpable landmarks are absent. Percutaneous localization can also be used for teaching blockade techniques to the novice and has been successfully used to demonstrate nerve or plexus blocks in a workshop setting.